Method and apparatus for establishing communication in a multi-tone data transmission system

ABSTRACT

Apparatus and methods for informing the central unit of the transmission requirements of a remote unit in a bi-directional data transmission system that facilitates communications between a central unit and a plurality of remote units using a frame based discrete multi-tone (DMT) transmission scheme. The method includes the step of transmitting, using a fast access transmission mode, a communication access request from a selected first remote unit to the central unit during any symbol period. The communication access request includes a unique remote unit identifier associated with the selected first remote unit and is transmitted from the selected first remote unit on at least one unused sub-channel using a modulation scheme that does not require equalization to decode at the central unit. The method further includes the step of allocating at least one sub-channel to the selected first remote unit in response to the communication access request for facilitating upstream communications between the selected first remote unit and the central unit.

This application is a Continuation-in-part of co-pending applicationSer. No. 08/377,023, filed Jan. 20, 1995, which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to discrete multi-tonecommunication systems in which a central unit services a plurality ofremote units. More specifically, it relates to methods for coordinatingupstream communications from the remote units.

Discrete Multi-Tone (DMT) data transmission schemes have been shown tofacilitate high performance data transmission. Among the benefits of DMTarchitectures is that they have high spectral efficiencies and canadaptively avoid various signal distortion and noise problems. Sincethey have very high data transmission capabilities, in most applicationsselection of a DMT data transmission scheme will provide plenty of roomfor the expansion of service as the demands on the data transmissionsystem increase. Hence, discrete Multi-Tone technology has applicationsin a variety of data transmission environments. For example, at the timeof this writing, the Alliance For Telecommunications InformationSolutions (ATIS), which is a group accredited by the ANSI (AmericanNational Standard Institute) Standard Group, is nearing finalization ofa discrete multi-tone based standard for the transmission of digitaldata over Asymmetric Digital Subscriber Lines (ADSL). The standard isintended primarily for transmitting video data over ordinary telephonelines, although it may be used in a variety of other applications aswell. The pending North American Standard is referred to as the T1E1.4ATIS Standard, and is presently set forth in Standard Contribution No.94-007, rev. 8, dated January of 1995, which is incorporated herein inits entirety.

Transmission rates under the ADSL standard are intended to facilitatethe transmission of information at rates of at least 6 million bits persecond (i.e., 6+Mbit/s) over twisted-pair phone lines. The standardizeddiscrete multi-tone (DMT) system uses 256 "tones" or "sub-channels" thatare each 4.3125 kHz wide in the forward (downstream) direction. In thecontext of a phone system, the downstream direction is generallyconsidered transmissions from the central office (typically owned by thetelephone company) to a remote location that may be an end-user (i.e., aresidence or business user). In other systems, the number of tones usedmay be widely varied. However when IFFT modulation is done, typicalvalues for the number of available sub-channels (tones) are integerpowers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels.

The Asymmetric Digital Subscriber Lines standard also contemplates theuse of a reverse signal at a data rate in the range of 16 to 800 Kbit/s.The reverse signal corresponds to transmission in an upstream direction,as for example, from the remote location to the central office. Thus,the term Asymmetric Digital Subscriber Line comes from the fact that thedata transmission rate is substantially higher in the forward directionthan in the reverse direction. This is particularly useful in systemsthat are intended to transmit video programming or video conferencinginformation to a remote location over the telephone lines. By way ofexample, one potential use for the systems allows residential customersto obtain video information such as movies over the telephone lines orcable rather than having to rent video cassettes. Another potential useis in video conferencing.

The discrete multi-tone (DMT) transmission scheme has the potential foruse in applications well beyond data transmissions over telephone lines.Indeed, DMT can be used in a variety of other digital subscriber accesssystems as well. For example, it may be used in cable based subscribersystems (which typically use coaxial cable) and wireless subscribersystems such as digital cellular TV. In cable systems, a single centralunit (central modem) is typically used to distribute digital signals tomore than one customer, which means more than one remote unit (remotemodem). While all of the remote modems can reliably receive the samedigital signals, the upstream transmissions must be coordinated toprevent confusion at the central modem as to the source of the upstreamsignals. In some existing cable systems (which do not use discretemulti-tone transmission schemes), each remote unit is given a dedicatedfrequency band over which it is to communicate with the central station.However, such an approach is inherently an inefficient use oftransmission bandwidth and typically requires the use of analog filtersto separate transmissions from the various remote units. Other existingcable systems use a single wide band for all remote units, which usetime division multiple access (TDMA) to access the upstream channel.This approach is inefficient because of the lower total capacity of thesingle channel and because of the time required for the accessingprocess. Stationary digital cellular transmission systems face similarobstacles. The ability to access the channel on both a time- andfrequency-divided basis would more efficiently utilize the transmissionchannel. However, the inherent multiplexing nature of DMT has previouslyrestricted its application to point-to-point transmission becausetransmissions from different sources must be synchronized for theall-digital multiplexing to function properly.

ADSL applications have the potential for a similar problem, although itis typically more limited in nature. Specifically, a single line mayservice a plurality of drop points at a particular billing address(which may typically be a home or an office). That is, there may beseveral telephone "jacks" through which the user may wish to receivesignals. To facilitate service to multiple locations (jacks) over asingle line, the use of a master modem has been proposed to facilitatesynchronization. However, this is perceived as being a relativelyexpensive and undesirable solution. Accordingly, it would be desirableto provide a mechanism in discrete multi-tone data transmission systemsthat facilitates the synchronization of signals from a plurality ofremotes so that a central unit can coordinate and reliably interpretsignals sent from the remotes.

One method of synchronization remote units utilizes the concept of adedicated overhead bus. That is, one or more dedicated overheadsub-channels are used to facilitate initializing new remote units. Thissystem is described in John M. Cioffi's co-pending U. S. patentapplication Ser. No. 08/252,829, which is assigned to the assignee ofthe present application and is incorporated herein by reference.Although the use of an overhead bus works well in some applications,other methods of coordinating multi-point to point transmission aredesirable as well.

Another feature of transmission systems currently utilized forcommunications from a remote unit to a central unit is that they eithertransmit data at a designated maximum rate (frequency-divisionmultiplexing), or they transmit data in packets of a particular size(time-based multiplexing). They do not permit both. This limits theefficiency of the use of the transmission channels. Accordingly, itwould be desirable to provide a mechanism through which when necessary,a remote unit can specify a desire to transmit at a particular data rateand when the data rate is not a concern, the remote unit may indicatethat it desires to transmit a designated amount of information.

SUMMARY OF THE INVENTION

In accordance with the purpose of the present invention, abi-directional data transmission system that facilitates communicationsbetween a plurality of remote units and a central unit using asymbol-based discrete multi-tone (DMT) transmission scheme is disclosed.The present invention provides several novel arrangements and methodsfor coordinating communications between a plurality of remote units anda central unit to facilitate multi-point-to-point transmission.

In one embodiment, the invention relates to a method of informing thecentral unit of the transmission requirements of a remote unit. Themethod includes the step of transmitting, using a fast accesstransmission mode, a communication access request from a selected firstremote unit to the central unit, the communication access requestincluding a unique remote unit identifier associated with the selectedfirst remote unit and being transmitted from the selected first remoteunit on at least one unused sub-channel using a modulation scheme thatdoes not require equalization to decode at the central unit. The methodfurther includes the step of allocating at least one sub-channel to theselected first remote unit in response to the communication accessrequest for facilitating upstream communications between the selectedfirst remote unit and the central unit.

In another embodiment, the communication access request further includesa data transmission request.

In yet another embodiment, the invention relates to an apparatus fortransmitting data from a selected remote unit to the central unit. Theapparatus includes a serial to parallel converter for receiving the dataand converting the data to parallel data. The apparatus also includes anencoder coupled to the serial to parallel converter for encoding theparallel data according to one of a first and a second modulationschemes responsive to a control signal, the first modulation schemebeing operative during a polled transmission mode and requires areceiver at the central unit to have prior knowledge of the identity ofthe selected remote unit for decoding, the second modulation schemebeing operative during a fast access transmission mode and does notrequire the receiver at the central unit to have prior knowledge of theidentity of the selected remote unit for decoding.

Further, the apparatus includes an IFFT modulator coupled to the encoderfor modulating encoded data from the encoder, and a parallel to serialconverter coupled to the IFFT modulator for convening modulated datafrom the IFFT modulator to a serial format for transmission to thecentral unit.

In accordance with one embodiment of the apparatus, the first modulationscheme is QAM and the second modulation scheme is DQPSK.

In yet another embodiment, the invention relates to an apparatus forreceiving data transmitted from a selected remote unit to the centralunit. The apparatus includes a serial to parallel converter forreceiving the data and converting the forward error corrected data toparallel data.

The apparatus further includes a FFT demodulator coupled to the serialto parallel converter for demodulating parallel data from the serial toparallel converter. Further, the apparatus includes a decoder coupled tothe FFT demodulator for decoding demodulated data from the FFTdemodulator according to one of a first and a second demodulationschemes responsive to a control signal, the first demodulation schemebeing operative during a polled transmission mode and requires priorknowledge of the identity of the selected remote unit for decoding, thesecond demodulation scheme being operative during a fast accesstransmission mode and does not require prior knowledge of the identityof the selected remote unit for decoding. Furthermore, the apparatusincludes a parallel to serial converter coupled to the decoder forconverting decoded data from the decoder to a serial format.

It should be appreciated that the various embodiments may be used eitherstanding alone or in combination with one or more of the others. Thedescribed systems may be used regardless of whether the downstreamsignals are also discrete multi-tone. In several preferred embodiments,the bi-directional data transmission system is a cable system thatincludes the transmission of signals over a coaxial cable, althoughother systems are contemplated as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is block diagram of a communication system including a head endcentral unit that services a plurality of remote units.

FIG. 2 is a frequency diagram illustrating the use of a multiplicity ofdelimited sub-channels used in a DMT system.

FIG. 3 is a timing diagram of a DMT data transmission system inaccordance with one embodiment of the present invention.

FIG. 4 is a flow diagram illustrating a method of initializing a remoteunit in accordance with one aspect of the present invention.

FIG. 5 is a flow diagram illustrating a method of retraining a remoteunit in accordance with a second aspect of the present invention.

FIG. 6 is a flow diagram illustrating the steps taken by a requestingremote unit to establish communication with a central unit.

FIG. 7(a) is a flow diagram illustrating a method of allocatingbandwidth to a remote unit making a data packet request.

FIG. 7(b) is a flow diagram illustrating a method of allocatingbandwidth to a remote unit making a defined data packet request.

FIG. 7(c) is a flow diagram illustrating a method of allocatingbandwidth to a remote unit making a data rate request.

FIG. 8 is a block diagram of a transmitter suitable for transmittingdigital signals using either the fast access transmission mode or thepolled transmission mode in accordance with the present invention.

FIG. 9 is a block diagram of a receiver suitable for receiving digitalsignals transmitted over a plurality of sub-channels using either thefast access transmission mode or the polled transmission mode.

DETAILED DESCRIPTION OF THE INVENTION

As described in the background section of this application, onelimitation of discrete multi-tone transmission systems is that in orderto support a plurality of drop points serviced by a single line, theupstream signals must be synchronized when they arrive at the centralunit. This synchronization problem has limited the attractiveness ofDiscrete Multitone (DMT) data transmission schemes in certainapplications such as cable systems and wireless cellular televisiondelivery since these systems use a single line (medium) to service arelatively large number of independent remote units, which wouldtypically be operated by different subscribers.

Referring initially to FIG. 1, a schematic transmission scheme for atypical multiuser subscriber network will be described. A central unit10 (which includes a central modem) communicates with a plurality ofremote units over a common transmission line 17 which is split into aplurality of feeds 18. Each feed 18 services an associated remote unitwhich typically includes a remote modem 15 which receives the signalsand a remote device 22 which uses the data. A service provider 19 wouldtypically be arranged to provide the data to the central modem fortransmission to the remote modems 15 and to handle the data received bythe central modem from the remote modems. The service provider 19 cantake any suitable form. By way of example, the service provider can takethe form of a network server. The network server can take the form of adedicated computer or a distributed system. A variety of transmissionmedia can be used as the transmission line. By way of example, twistedpair phone lines, coaxial cables, fiber lines and hybrids thatincorporate two or more different media all work well. This approachalso works well in wireless systems.

As will be appreciated by those skilled in the art, one requirement ofdiscrete multitone data transmission systems such as those contemplatedherein is that if two or more units (typically two remote units) areattempting to independently transmit information to a third unit (i.e.the central unit 10), the signals from the remote units must bysynchronized or at least some of the signals will be incomprehensible tothe central unit 10. The problem with using discrete multi-tonetransmissions in such a system is that the length of the feeds 18 willtypically vary from remote to remote. Therefore, even if the remotessynchronize with the clock of the central unit 10, their communicationsback to the central unit 10 will be phase shifted by an amount that isdependent at least in part on the length of the associated feed. Inpractice, these types of phase shifts can make remotely initiatedcommunications unintelligible to the central modem.

As is also well known to those skilled in the art, DMT transmissioninherently partitions a transmission medium into a number ofsub-channels 23 that each carry data independently. The data on eachsub-channel 23 can correspond to a different signal or can be aggregatedinto higher data rates that represent a single or fewer wider-bandwidthtransmissions. These sub-channels 23 are implemented entirely withdigital signal processing in DMT, which eliminates the need for analogseparation filters and maximizes spectral efficiency. A representativeDMT transmission band is illustrated in FIG. 2. As seen therein, thetransmission band includes a multiplicity of sub-channels 23 over whichindependent carrier signals (referred to as sub-carriers 27) may betransmitted. The number of sub-channels used may be widely varied inaccordance with the needs of a particular system. However, whenmodulation is performed using an Inverse Fast Fourier Transform (IFFT),typical values for the number of available sub-channels 23 are integerpowers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels23. By way of example, in one embodiment that is adapted for use in acable based subscriber system, 1024 sub-carriers 27 may be used witheach carrier confined to a 32 kHz sub-channel 23. This providesapproximately 32 MHz of frequency bandwidth in which the remote unitscan communicate with the central unit 10.

The number of remote units that may be used in any particular system mayvary greatly in accordance with the needs of a particular system. By wayof example, in one embodiment of the described cable based subscribersystem, it may be desirable to permit up to 500 remote units tocommunicate with a single central unit. In systems that contemplate sucha large number of remote units, it may be desirable to allocate theremote units in groups. Of course, the groups need not each contain thesame number of units. By way of example, a system that permits up to 500remote units may divide the remote units into eight groups, with eachgroup permitting up to 90 remote units, with each remote unit groupbeing assigned a designated frequency band. For example, the frequencyspectrum may be divided into a plurality of equally sized designatedfrequency bands. In the particular embodiment described, one-eighth ofthe 32 MHz, or approximately four megahertz would be assigned to eachgroup. Therefore, each group would have 4 MHz, and correspondingly, 128sub-channels 23 to use for transmitting to the central unit 10. Groupingallows the central unit 10 to keep track of the remote units in amanageable manner as they come on and off line.

The groupings can be made using any number of methods. By way ofexample, a first group could consist of consecutive sub-channels 0-127,a second group sub-channels 128-255 and so forth. Alternatively, theallocation of sub-channels 23 to the respective groups may beinterleaved throughout the spectrum. For example, the first group may beassigned sub-channels 0, 8, 16, 24, 32 . . . ; the second group may havesub-channels 1, 9, 17, 25, 33 . . . ; the third group: 2, 10, 18, 26, 34. . . ; and so forth. The interleaving of sub-channels 23 assigned tothe groups helps to reduce the probability that noise located in oneparticular area of the frequency spectrum will corrupt a significantportion of the transmissions in a single group. Instead, the spuriousnoise will affect only a portion of the spectrum for each group. As canbe appreciated by those skilled in the art, the frequency bandwidth ofthe upstream channel, size of the sub-channels 23 and the groupings arenot restricted to the numbers in the described embodiment but can bechosen to suit the needs of the particular use of the transmissionsystem.

As will be described in more detail below, in one aspect of the presentinvention, synchronized quiet times are periodically provided in theupstream communication stream. The synchronized quiet times may be usedto handle a variety of overhead type functions such as initialization ofnew remote units, transmission channel quality checking and handlingdata transfer requests. Referring next to FIG. 3, a representative framedelimited transmission timing sequence is illustrated that provides anumber of synchronized quiet periods that are suitable for handling theoverhead functions. In the embodiment shown, the transmissions arebroken up into string of transmission frames 32. Each transmission frameincludes a transmission interval 33 and a first quiet interval S1. Eachtransmission interval 33 is further divided into a plurality of symbolperiods 35 as shown. A plurality of transmission frames 32 are thengrouped together into a super-frame 36. In addition to the transmissionframes 32, each super-frame 36 also includes a second quiet timeinterval 38. In the embodiment described, the second quiet time interval38 may be used as either an initialization interval (S2) or a retraininginterval (S3).

The actual periods provided for the transmission interval 33, the quiettime interval S1, the initialization interval S2 and the retraininginterval S3 may be widely varied in accordance with the needs of aparticular system. Similarly, the number of transmission frames 32 in asuper-frame 36 may be widely varied. By way of example, one suitableembodiment for use in the described cable-based subscriber system,contemplates a transmission interval 33 set to a period sufficient totransmit 63 symbols and the S1 time interval 34 set to one symbol inlength of time. The length of the second quiet time interval 38 istypically determined by the physical aspects of the communicationssystem, as will be discussed in more detail below. In general, theremote units are required not to broadcast during an S1 or S3 quiet timeinterval unless given permission by the central unit 10. The remoteunits are also required not to broadcast during an S2 quiet timeinterval unless they are seeking to initiate installation as will bedescribed in more detail below.

Referring to FIG. 4, a method of initializing a first remote unit duringinstallation that utilizes the described second quiet times S2 inaccordance with one aspect of the invention will be described. When aremote unit first comes on line it must be initialized such that thetransmissions from the first remote unit are synchronized with thetransmissions of any other currently installed remote units. That is,the frame boundaries of upstream DMT communications from the variousremote units to the central unit must be substantially synchronized atthe central unit for the transmissions to be understood by the centralunit. The method described with reference to FIG. 4 is one method ofaccomplishing such synchronization utilizing the described quiet times.

Initially, the remote unit to be installed must establish a connectionto the transmission network in step 102. The connection enables theremote unit to listen to the downstream transmissions from the centralunit 10 and transmit on any sub-channel 23 of the upstream channel. Insome systems, there may be certain frequency ranges that the system maynot use. By way of example, in many cable networks there may beestablished systems that utilize specific frequency bands. In order toprevent interference and maintain backward compatibility, it isimportant that the remote unit never transmit in the forbidden frequencyrange, even during initialization. Of course, certain frequency bandsmay be forbidden for other reasons as well. Accordingly, in step 103,the central unit will periodically broadcast an identification offrequencies that may never be used. In systems that utilize the conceptof remote unit groups as discussed above, the central unit may alsoperiodically broadcast the group number of the group that should be usedby the next remote unit to be installed. Alternatively, the groupassignment can be handled at a later point.

The newly connected remote unit listens to the downstream signals forinformation indicating that certain sub-channels may not be used. Thedownstream signal also includes the frame timing and quiet periodmarkers required to synchronize the remote unit with the central unit.After the remote unit has synchronized itself with the downstreamsignal, in step 104 it transmits an initialization signal at thebeginning of an S2 quiet period. In one system, this is done bytransmitting an initialization signal immediately upon receiving an S2quiet period marker signal. The initialization signal indicates to thecentral unit 10 that a remote unit requests to be installed onto thesystem. The remote unit may determine the onset of an S2 initializationquiet period in any suitable manner. By way of example, a flag may beprovided by the central unit 10 in the downstream communications. Theremote unit may transmit its initialization signal over all thesub-channels 23, over a group of sub-channels 23 or on a singlesub-channel 23 depending on the needs of a particular system. In apreferred embodiment, the downstream signal indicates the group to beused by the next unit to be installed, and the initialization signal istransmitted over all the sub-channels in that group.

The upstream initialization transmissions from the remote units to thecentral unit 10 can be accomplished in any modulation scheme suitablefor transmitting digital information. By way of example, amplitude,frequency, and quadrature phase shift key (QPSK) modulation schemes canbe utilized. For the synchronization signal, differential QPSK (DQPSK)modulation is desired in a preferred embodiment to decrease thepossibility of corruption by noise. Additionally, the synchronizationcan be encoded with a large amount of error correction and redundancy toensure coherent communications.

The initialization signal preferably contains information about theremote unit. In a preferred embodiment the initialization signal carriesthe global address of the remote unit and the maximum transmission datarate requirement of the first remote unit. A global address, is similarto addresses used on ethernet or cellular devices. Such addresses arebuilt into the communications device and are distinct from addresses ofall other communicating devices. The maximum data rate required by theremote unit is dependent upon the type of device the remote unit is. Forexample if the remote unit is a television set it would require minimalcommunications capacity to the central unit 10, possibly only using theupstream signals to send information about movie selections or listenerfeedback. On the other hand, if the remote unit is a teleconferencingtransceiver then a large amount of bandwidth would be required totransmit video and audio information from the remote unit to the centralunit 10. Other pieces of relevant information about the first remoteunit can also be sent along with the initialization signal in otherembodiments.

Upon receiving the initialization signal from the first remote unit, thecentral unit 10 determines in step 106 whether the initialization signalfrom the first remote unit has collided with another initializationsignal from another remote unit trying to connect at the same time. If acollision is detected then the central unit 10 transmits a collisionmessage back to the remotes in step 108. The collision message indicatesto the remote units trying to connect to try again. The colliding remoteunits then each wait a random number of S2 periods before re-sending aninitialization signal. The probability of two remote units trying toinitialize at the same time is small. By requiring the colliding unitsto wait random amounts of time that are independent of each other, theprobability of repeat collisions is reduced even further.

After the central unit 10 receives a valid initialization signal fromthe first remote unit, the central unit 10 transmits a synchronizationsignal 110 back to the remote unit. In one embodiment, thesynchronization signal includes the global address of the first remoteunit, a nodal address assigned to the first remote address, delaycorrection information, and information about the allocation of thesub-channels 23 in the upstream channel. Either the global address andthe nodal address can serve as a unique remote unit identifier, albeitwith differing degrees of transmission efficiency. The global addressallows the first remote unit to identify that the synchronization signalis intended for it. The nodal address is assigned to the first remoteunit in order to facilitate efficient communications. The global addresscan be quite long (as for example 48 bits) to allow for an adequatenumber of global addresses for all the communicating devices that arelikely to be manufactured. The nodal address is a shorter address sinceonly a limited number of remote units will be communicating with anysingle central unit 10. When a multi-grouped system is used, the nodaladdress also contains group identifier information, e.g. informationabout the group to which the first remote unit is assigned. In theembodiment described above which contemplates a total of eight groups,that part of the address would be three bits to identify which of theeight groups the first remote unit is in. The remainder of the bits canuniquely identify the node, e.g. the specific remote unit, within itsgroup.

It should be appreciated by those skilled in the art that the part ofthe nodal address that specifies the group, i.e. the group identifierinformation, may be omitted altogether when a remote unit needs touniquely identify itself to the central unit. This is because thecentral unit may, by inspecting the frequency band of the uniqueidentifier message, determine the group from which the remote unit'smessage is sent. In this manner, a remote unit needs to send only thebit pattern in the nodal address that identifies itself in the group,i.e. the unique intra-group identifier information, in order to uniquelyidentify itself to the central unit. This received intra-groupidentifier bit pattern, in combination with the ascertained groupidentifier information, provides the central unit with the completenodal address of the requesting remote unit. In the preferred embodimentwhich has 128 sub-channels per group, the unique remote identifierinformation may be as short as 7 bits in the upstream direction.

The delay correction information tells the first remote unit how muchthe frames being broadcast from the first remote unit must be delayed inorder to synchronize them with signals from the other connected remoteunits. The delay correction is determined from the amount of delay thatthe central unit detects between the time it transmits a quiet period(S2) marker and its reception of the initialization signal. By way ofexample, if the maximum delay in the channel is T_(RT) (Max), e.g.maximum round-trip delay, and the delay associated with a given remoteunit is T_(RT) (i), the delay correction for that remote unit is T_(RT)(Max)-T_(RT) (i). The round-trip delay for a remote unit is defined asthe time taken for a signal to travel from the central unit to thatremote unit, and an immediate response to be returned to the centralunit, including any minimal, incidental delay attributable toprocessing. Using this information the first remote unit can adjust itstransmissions and become synchronized with the other connected remoteunits, such that the frames of the remote units arrive at the centralunit 10 at the same time. The first remote unit may also learn whichsub-channels 23 are currently in use by the other connected remoteunits. In another embodiment, information about sub-channel 23characteristics are regularly transmitted to all the remote unitsthrough the downstream channel. In such systems, channel usageinformation would not be required to be sent along with thesynchronization signal.

One advantage of transmitting the initialization signals over a broadportion of the available spectrum is that delays may vary to some extenddepending upon the frequency at which the signal is transmitted.Therefore, when the initialization signals are transmitted over avariety of the sub-channels 23 the required phase shift can becalculated based on an average of the individual delays.

The length of the S2 time interval, as discussed earlier, is dependentupon the physical nature of the communications network. In a preferredembodiment the S2 time interval need only be longer than the duration ofthe initialization signal plus the difference between the maximum andminimum round-trip delays for the network. By way of example, in atypical system employing a fiber optic trunk as the transmission line 17and coaxial cables as the feeds 18, the fiber trunk is common to allpaths between central and remote units, and the difference between themaximum and minimum round-trip delays for the network depends only onthe cable part of the network. Using the length of 2 miles for thecoaxial line and given its propagation time of approximately 7.5microseconds per mile, the maximum and minimum roundgrip delays would beapproximately 32 and 2 microseconds. In a preferred embodiment a symbolis approximatley 30 microseconds long, and an initialization signalwould comprise two symbols, so that, by way of example, an S2 timeinterval of 4 symbols would be appropriate.

In certain embodiments, it may be desirable to repeat steps 104-110 tovalidate the information received and/or ensure that the remote isproperly synchronized.

After synchronization has been accomplished, the first remote unitresponds by sending a set of synchronized wide band training signalsover all the sub-channels 23 during the next available S2 or S3 timeinterval in step 112. The specifics of the training step will bedescribed in more detail below with reference to FIG. 5. In someembodiments, the central unit 10 will direct the first remote unit touse a specified S3 time interval (e.g., wait for the third S3). Uponreceipt of the training signals, the central unit 10 determines thecapacities of the various sub-channels 23 to handle transmission betweenthe first remote unit and the central unit 10 (step 114). The centralunit 10 preferably has a prior knowledge of the contents of the trainingsignals. This allows the central unit 10 to learn the optimalequalization of the sub-channels 23 and also the maximum bit rates asubcarrier 27 can carry on the sub-channels 23 between the first remoteunit and the central unit 10. The central unit 10 saves the channelcharacteristics of the sub-channels 23 with respect to the first remoteunit 116. In a preferred embodiment the central unit 10 saves theinformation in a bits/carrier matrix that contains an indication of thenumber of bits that each of the sub-channels 23 can carry to each of theremote units. Such a matrix allows the central unit 10 to keep track ofthe capacity of each of the various sub-channels 23 and is availablewhen allocating bandwidth to the remote units. This also facilitates thedynamic allocation of sub-channels based upon the currentcharacteristics of the transmission environment.

Referring next to FIG. 5, a method of periodically checking the capacityof the various sub-channels from a selected remote unit to the centralunit will be described. As will be appreciated by those skilled in theart, the capacity of the transmission line at various frequencies mayvary somewhat over time. Therefore, it is desirable to periodicallyupdate the central unit's information concerning the characteristics ofthe sub-channels 23 with respect to each of the remote units itservices. In the embodiment described, such updating is done during theS3 quiet periods. In the embodiment shown, the S3 quiet periods are ofthe same length as the S2 quiet periods. It should be appreciated that asingle transmission line checking process may be used both for theinitial training and the periodic checking.

In the described embodiment, the central unit 10 initiates a retrainingevent in step 130 by transmitting a retraining command to a first remoteunit (remote unit x) that is in current communication with the centralunit 10. The first remote unit waits for the next available S3retraining quiet time interval to transmit a set of training signalsover the available sub-channels 23. (Step 132). In an alternativeembodiment, the central unit 10 may assign a specific S3 quiet intervalto use for transmitting the training signals, instead of the nextavailable S3 time interval. The set of training signals will typicallybe limited to the sub-channels allocated to the group and will typicallybe further limited to some subset of the total available groupsub-channels to provide a cost effective design. Therefore, the numberof training signals that are actually used may be widely varied inaccordance with the needs of a particular system. As in theinitialization process, the central unit 10 analyzes the signals itreceives and updates the bit/carrier rates in the channelcharacteristics matrix that correspond to the associated remote unit.(Step 134). The central unit 10 then determines whether a change in thesub-channel allocation is necessary for the remote unit. That is, it maydetermine whether additional or fewer sub-channels 23 should beallocated to the first remote unit in order to meet the first remoteunit's throughput and error probability requirements. If a change isnecessary, then the central unit 10 re-allocates sub-channels 23 to thefirst remote unit in step 138.

If it is determined that no correction is required in step 136 or afterany necessary changes have been made in step 138, the central unit 10checks to see if there have been any requests made by any other remoteunits for an immediate retraining in step 140. If it is determined instep 140 that there are no immediate retraining requests, the centralunit 10 checks to see if the retraining of the first remote unit was aresult of a immediate retraining request by checking if there is a validold address (oldx) in step 147. If there is no valid old address thenthe central unit 10 increments the counter (x) in step 149 and returnsto step 130 where it broadcasts a retrain signal to the next remoteunit. On the other hand, if it is determined in step 140 that there wasa valid old address, the central unit 10 will adjust the counter suchthat it reads one more than the old address, which corresponds to theaddress of the remote unit that would have been next at the time animmediate retrain request was received. (Step 150). That is, x=oldx+1.

If an immediate retrain request was detected in step 140, then thecentral unit 10 saves the address of the first remote unit as an oldaddress (oldx) in step 142. The central unit 10 then sets the counter(x) to the address of the requesting remote unit and uses it as theaddress of the next remote unit currently being retrained 144. The logicthen returns to step 130. The retraining process may then be continuallyrepeated among all the remote units currently communicating with thecentral unit 10. Of course, the algorithm used to select the remoteunits for retraining may be widely varied to meet the requirements ofany particular system.

In one embodiment, the remote units that have been initialized but arenot currently communicating with the central unit 10 are also retrained.In that case, the central unit 10 need not determine if the allocationof sub-channels 23 has to be changed for the remote unit being retrainedsince it is not actively communicating with the central unit 10. Thecentral unit 10 can merely save the updated channel characteristics tobe used when the remote unit requests communication with the centralunit 10.

The central unit 10 is preferably adapted to receive a retrainingrequest on unused sub-channels 23 during a transmission time interval32. In a preferred embodiment, the transmission time interval 32 is 64symbols long, corresponding to the maximum number of possible remoteunits within a group. A remote unit requiring an immediate retrainingtransmits a flag during one of the symbol times assigned to therequesting remote unit in the transmission time interval 32. In thismanner, the central unit 10 can immediately determine which remote unitsent the request by the location of the flag. For example, remote units0-63 in group eight may be assigned symbols 0-63 respectively in thetransmission time interval. If a flag arrives on an unused sub-channel23 in the group eight frequency band during the ninth symbol position,then the central unit 10 knows that the ninth remote unit in group eighthas sent a retraining request. As can be appreciated by those skilled inthe art, the assignment of remote units to symbols can be accomplishedin many different ways.

As discussed above, in order to facilitate a dynamically allocateddiscrete multi-tone transmission scheme, there must be some mechanism bywhich the remote units can communicate a data transmission request tothe central unit. In one embodiment, the S1 quiet times are used inconjunction with a data transmission request to facilitate initiation ofa transmission. In the described embodiment, a remote unit may sendthree types of data requests to the central unit. They include a datapacket request (DPR), a defined data packet request (DDPR) and a datarate request (DRR). As used in this embodiment, a data packet requestindicates the remote unit's desire to transmit a specific volume ofinformation (which is typically defined in terms of a number of databytes). A defined data packet request indicates the remote unit's desireto transmit a packet or group of packets having characteristics alreadyknown to the central unit. By way of example, the central unit mayalready have stored in its memory the information regarding the remoteunit to which data packets from the requesting remote unit should besent. Other information known to the central unit may include, forexample, the transmission rate for the data packets, the number ofsub-channels needed by the requesting remote unit, and the like. A datarate request indicates the remote unit's desire to transmit data at aparticular rate.

The described data transmission requests may, in one embodiment, becoupled with the immediate retrain request described above in a simpletwo bit signal that includes four states. By way of example, one state(1,1) may correspond to a Data Rate Request; a second state (1,0) maycorrespond to a Data Packet Request, a third state (0,1) may correspondto an immediate retrain request, and a fourth state (0,0) may correspondto a Defined Data Packet Request. Of course, the same information can beincluded as part of a larger signal and/or the meaning of the variousstates may be varied. As described above, the two bit data transmissionrequest signal may be transmitted by a remote unit over sub-channelsthat are not in use. By assigning a particular symbol period to eachremote unit, the central unit can readily identify the requesting remoteunit without requiring any independent identification information in thedata transmission request signal. This transmission mode, which assignsa particular symbol period to each remote unit, is termed the polledtransmission mode.

As will be appreciated by those skilled in the art, in addition tomerely identifying the type of information the remote unit wishes totransmit, in the case of both the Data Rate Request and the Data PacketRequest, the remote will normally need to provide substantially moreinformation to the central unit in order for the central unit toproperly handle the request. In order to provide quick access times, theextra information is relayed to the central units during the nextavailable S1 quiet time interval. More specifically, when the centralunit 10 receives a valid data packet request or a valid data raterequest, the central unit 10 directs the requesting remote unit totransmit any additional information about the requesting remote unit'srequest during the next available S1 quiet period 34. During the S1quiet period, the requesting remote unit has access to as manysub-channels as it needs to transfer the header information. Since boththe Data Rate Request and the Data Packet Request effectively requestonly the allocation of an S1 quiet period, they could readily share asingle state in the two bit data transmission request signal.Accordingly, in alternative embodiments, a single state could beprovided to indicate the desire for allocation of a S1 quiet period andthe nature of the request could be transmitted during the S1 periodalong with the other information.

When the system is not being heavily used, there may be a relativelylarge number of sub-channels that are available to the remote unit whenit sends its data transmission request. During such periods, it may bepossible to transmit all of the required header information concurrentwith the transmission of the data request in the same symbol period.Thus, in one alternative embodiment, the free state in the datatransmission request may be used to flag to the central unit that theremote unit is transmitting the required header information on unusedsub-channels simultaneously with the data transmission request. In thepolled transmission mode, the timing of the data transmission requestwould identify the remote unit sending the request. Thus, the advantageof this approach is that during times of relatively light usage, theaccess times for data rate and data packet requests may be even furtherreduced. Conflicts would not occur between two remote units since eachremote only transmits during its assigned symbol period. When the remoteunit determines that there is not enough bandwidth to accept all of therequired header information in the assigned symbol period, it wouldsimply request allocation of an S1 quiet period as described above.

In another embodiment, the central unit 10 can assign a specific S1interval 34 for the requesting remote unit to use. This is especiallyuseful when two or more remote units simultaneously make data packet ordata rate request.

When the system is not being heavily used, there may be a relativelylarge number of sub-channels that are available to the remote unit whenit sends its data transmission request. During such periods, it may bepossible to transmit all of the required header information concurrentwith the transmission of the data request in the same symbol period.Thus, in one alternative embodiment, the free state in the datatransmission request may be used to flag to the central unit that theremote unit is transmitting the required header information on unusedsub-channels simultaneously with the data transmission request. In thepolled transmission mode, the timing of the data transmission requestwould identify the remote unit sending the request. Thus, the advantageof this approach is that during times of relatively light usage, theaccess times for data rate and data packet requests may be even furtherreduced. Conflicts would not occur between two remote units since eachremote only transmits during its assigned symbol period. When the remoteunit determines that there is not enough bandwidth to accept all of therequired header information in the assigned symbol period, it wouldsimply request allocation of an S1 quiet period as described above.

In another embodiment, the central unit 10 can assign a specific S1interval 34 for the requesting remote unit to use. This is especiallyuseful when two or more remote units simultaneously make data packet ordata rate request.

As noted earlier, when the system is not being heavily used, there mayexist a relatively large number of sub-channels that are unused andavailable to a remote unit for requesting access. When the central unitdetermines that usage in the system is light, say when usage falls belowa predefined usage threshold, the central unit may issue a command toall remote units to enable remote units to transmit their communicationaccess requests to the central unit using a fast access transmissionmode. Fast access transmission mode differs from the above describedpolled transmission mode in which each remote unit is assigned to asymbol period for the purpose of transmitting its data transmissionrequest signal. As the name implies, fast access transmission modesubstantially improves a requesting remote unit's access speed bypermitting a requesting remote unit to transmit a communication accessrequest on one of the unused or unallocated sub-channels during anysymbol period, regardless whether that symbol period has been assignedto it. The remote units know which sub-channels are unused because, forexample, the central unit monitors sub-channel usage and broadcastsinformation regarding sub-channel usage from time to time to all remoteunits.

Because a remote unit no longer has to wait until its assigned symbolperiod to assert a communication access request, it can assert itscommunication access request as soon as need arises. On the other hand,the timing of the request in the fast access transmission mode does notfurnish information regarding the identity of the requesting remoteunit. To identify which remote unit asserts a received communicationaccess request signal, fast access transmission mode therefore requiresthat each requesting remote unit sends a unique remote unit identifierupon requesting access. As mentioned earlier, the unique remote unitidentifier may be as few as 7 bits for systems having 128 sub-channelsper group.

In one embodiment, the communication access request signal includes adata transmission request. As mentioned previously, the datatransmission request identifies the type of data request, e.g. DPR,DDPR, or DRR, desired by the remote unit. If two bits are used foridentifying a data transmission request, the last state may be used toindicate whether the header data is simultaneously sent in the samesymbol period or during the following S1 period. Obviously if the datarequest is DDPR, there may be no header information since the centralunit may already know the transmission requirements, e.g. thedestination of the data packet, the packet size, the priority rating,and the like, associated with a particular remote unit. If the datarequest is DPR or DRR, the last state defined by the two-bit datatransmission request is examined by the central unit to determine whenheader information is sent.

In another embodiment, the communication access request further includesthe header information for DRR and DPR data requests. The inclusion ofthe header information increases the number of bits sent in the fastaccess transmission mode. When the number of bits increases, the chancefor a collision increases. Collisions occur when two remote unitssimultaneously assert their communication access requests on the sameunused sub-channel. Consequently, the preferred embodiment preferablykeeps the number of bits sent in the fast access transmission mode aslow as possible in order to minimize collisions. As is apparent, thefast access transmission mode is most suitable for DDPR data requestssince it is not necessary to send header information from the remoteunit to the central unit.

Therefore, a communication access request preferably includes only theremote unit's unique remote unit identifier and the two-bit datatransmission request. In one embodiment, however, if a communicationaccess request does not include the two-bit data transmission request,the central unit may assume that a DDPR data request is desired andproceed to allocate sub-channels to the requesting remote unit based onthe stored data packet defining information associated with that remoteunit.

Fast access transmission mode preferably requires that the communicationaccess request be transmitted from the remote unit to the central unitusing a modulation method that does not require equalization duringdecoding. Equalization is necessary in certain modulation schemes thatrequire the central unit to know about the characteristics of thesub-channel and the remote unit, e.g. the absolute amplitude of thereceived signal and the phase in order to decode incoming data.Obviously, when a communication access request arrives at the centralunit during fast access transmission mode, the central unit does notknow prior to decoding the identity of the requesting remote unit. Thisis because in fast access transmission mode, a remote unit may assertits communication access request during any symbol period, and thetiming of the request does not furnish information regarding theidentity of the requesting remote unit.

Since the identity of the requesting remote unit is not known prior todecoding, the communication access request cannot be decoded bymodulation methods that require prior knowledge of the sub-channel andthe remote unit identity, e.g. QAM. In one embodiment, the presentinvention advantageously encodes a remote unit's communication accessrequest using Differential Quadrature Phase Shift Keying (DQPSK). WhenDQPSK is used, the information regarding a communication access requestis stored in the differences in phase instead of in the absolute phase.Further, it is possible to choose an appropriate constellation such thatthe amplitude is irrelevant. In this manner, a communication accessrequest may be received and decoded by the central unit withoutrequiring prior knowledge of the identity of the requesting remote unit.

As mentioned earlier, fast access transmission mode does not require therequesting remote unit to wait until its assigned symbol period torequest access. Consequently, the access time may be as low as the timeit takes to send the communication access request plus the time it takesfor the central unit to send to the requesting remote unit informationallocating sub-channels for use by the requesting remote unit.

In one embodiment, fast access transmission mode is enabled by thecentral unit when system usage is light, e.g. below a predefined usagethreshold. Enabling fast access transmission mode during these timesreduces the chance of collisions since there are more unusedsub-channels on which one or more remote units may assert communicationaccess requests. If a collision occurs, the central unit receivesgarbled data, e.g. data that cannot be decoded. Without knowing whichremote unit requires access, the central unit therefore cannot allocatesub-channels to the appropriate requesting remote unit. In this case, arequesting remote unit may wait for a predefined time period afterasserting its communication access request and if no allocation occurs,it then retransmits the communication access request, preferably afterwaiting a random time period to reduce the probability of anothercollision. In one embodiment, if the central unit receives a garbleddata transmission on any unallocated or unused sub-channel, it assumesthat a collision between two or more communication access requests hasoccurred and broadcasts to all remote units a "collision detected"message to urge the remote units to resend its communication accessrequests, preferably after waiting a random time period.

As is apparent, when there is a large number of collisions, sub-channelusage may increase because of the resending activities by the remoteunits and, in one embodiment, the broadcast activity of the centralunit. If too many collisions occur, system usage may exceed thepredefined usage threshold, causing the central unit in one embodimentto issue a control command to all remote units to cease datatransmission in the fast access transmission mode and to resume datatransmission in the polled transmission mode, in which each remote unitonly transmits its data requests during its assigned symbol period.

FIG. 6 is a flow diagram illustrating the steps taken by a requestingremote unit to establish communication with a central unit. Referringnow to FIG. 6, there is shown a start step 160. From start step 160, themethod proceeds to step 162 where the requesting remote unit ascertainswhether the transmission mode is fast access or polled. If therequesting remote unit ascertains that the polled transmission mode iscurrently operative, e.g. responsive to a control signal from thecentral unit when system usage is heavy, the method proceeds to step 166to transmit data in the polled transmission mode. In the polledtransmission mode, the requesting remote unit only transmits its datarequest during its assigned symbol period on one or more unusedsub-channels.

On the other hand, if the requesting remote unit ascertains that thefast access transmission mode is currently operative, e.g. responsive toa control signal from the central unit when system usage is light, themethod proceeds from step 162 to step 164 to transmit its communicationaccess request on one or more unused sub-channels during any symbolperiod. As explained earlier, the requesting remote unit does not haveto wait until its assigned symbol period to transmit its communicationaccess request in the fast access transmission mode.

From either step 164 or 166, the method proceeds to step 168 todetermine whether the data request is a data packet request (DPR). If itis, the method proceeds to step 170 where the steps of FIG. 7(a) areexecuted. On the other hand, if the data request is not a DPR (asdetermined in step 168), the method proceeds to step 172 to determinewhether the data request is a defined data packet request (DDPR). If thedata is request is a DDPR, the method proceeds to step 174 where thesteps of FIG. 7(b) are executed. On the other hand, if the data requestis not a DDPR (as determined in step 172), the method proceeds to step176 to determine whether the data request is a data rate request (DRR).If the data is request is a DRR, the method proceeds to step 178 wherethe steps of FIG. 7(c) are executed. If the data request is none of theabove, the method proceeds to step 180 where the steps of FIG. 6 end. Itshould be appreciated that certain embodiments may include additionaldata request types and that the method may be adapted to proceed tohandle those additional data requests as appropriate. The adaptation ofthe disclosed method to handle specific additional data request typesare within the abilities of one skilled in the art given thisdisclosure.

Referring to FIG. 7(a) a method of handling a data packet request willbe described in more detail. Initially, the central unit 10 allocatesthe next available S1 time interval 34 to the requesting remote unit andforwards a message verifying the allocation with the downstream signal(Step 204). Then in step 206, the requesting remote unit transmits theadditional information during the allocated S1 time interval 34. By wayof example, the additional transmission requirements may include theaddress to which the data is being sent, the packet size, and a priorityrating. As discussed earlier, the remote unit may alternatively transmitthe additional transmission requirements in the same symbol period asthe transmission request.

The central unit 10 then stores the additional data packet informationthat it receives in step 208. The central unit 10 then determines thenumber of sub-channels that should be allocated for the remote unitsrequests and transmits instruction as to the sub-channels that are to beused together with the allowable bit rates per channel back to therequesting remote unit. It should be appreciated that the central unit10 will allocate sub-channels 23 based upon the stored set of channelcharacteristics that correspond to the requesting remote unit 210. Inthis manner the central unit 10 can dynamically allocate the mostefficient number of sub-channels 23 to handle the remote unit's request.It should be appreciated that the central unit receiver knows the amountof data to be transmitted (from the information received during the S1quiet period), as well as the data transmission rates (which the centralunit has specified). Therefore, the central unit knows the amount oftime that is needed to complete the transmission. Accordingly, thecentral unit 10 allocates the designated number of sub-channels 23 tothe requesting remote unit only for the amount of time required for therequesting remote unit to transmits its packet(s). After the specifiedamount of time has elapsed (with any necessary buffer), the central unit10 makes note that the sub-channels 23 assigned to the first remote unitare now unused and ready to be re-allocated to any other remote unit.(Step 212).

Referring next to FIG. 7(b), a method of handling a defined data packetrequest (DDPR) will be described. In a defined data packet request, thecentral unit must rely on the additional data packet defininginformation that was stored in step 208. Again, this may include suchthings as the address to which the packet(s) is being sent and thepacket size. Thus, in the described embodiment, a defined data packetrequest can be handled only if it is transmitted by a remote unit thathas previously sent a DPR. In alternative embodiments, appropriatedefaults could be provided to permit the use of defined data packetseven when no data packet request has been sent.

As illustrated in FIG. 7(b), in step 223, the central unit looks up thestored defined data packet transmission requirements and uses thatinformation in directing and/or handling the data packet(s) received. Itshould be appreciated that the central unit 10 does not need to receiveany additional information either in the same symbol period or during anS1 time interval 34 and therefore can immediately allocate one or moresub-channels 23 to the requesting remote unit in step 225. Again, sincethe amount of information to be transmitted and the data transmissionrates are both known, the central unit only allocates the sub-channelsfor the amount of time necessary to transmit the package. After theappropriate transmission time has elapsed, the central unit 10 notesthat the sub-channels 23 are now free to be re-allocated in 227.

While many communicating devices can effectively communicate throughpacketized communications, others require a constant rate oftransmission that is sometimes difficult to obtain using packetizedtransmission systems. Such remote units can be accommodated byallocating a number of sub-channels 23 that is sufficient for handlingthe required data transmission rate for an indeterminate amount of time.That is, until the remote unit indicates that the bandwidth is no longerrequired or an error is detected. By way of example, video conferencingis likely to have such requirements. In the described embodiment, thistype of data transmission request is handled through the use of a datarate request.

Referring next to FIG. 7(c), a method suitable for handling data raterequests will be described. The central unit 10 will typically requireadditional transmission information such as address and the requesteddata rates upon receiving a DRR request. Accordingly, in step 252, thecentral unit allocates the next available S1 quiet period to therequesting remote unit to send the required information. The requestingremote unit then sends the additional transmission information duringthe allocated S1 time interval in step 254. As discussed earlier, theremote unit may alternatively transmit the additional transmissionrequirements in the same symbol period as the transmission request.

Knowing the data rate requirements as well as the permissible bit ratesfor each subcarrier, the central unit 10 allocates an appropriate numberof sub-channels 23 to handle the requested throughput in step 256. Whenthe requesting remote unit no longer needs to transmit, it sends a newdata rate request indicating that zero capacity is required in step 258.The central unit 10 understands this as a termination request and marksthe appropriate sub-channels as unused in step 260.

There is no set period that is ideal for repeating the S1 quiet periods.On the one hand, the more frequent the S1 quiet periods, the shorter theaccess times that can be achieved for the polled transmission mode orfor DPR and DRR requests will be. Thus, the more responsive the systemwill be. On the other hand, more frequent S1 quiet periods require moreoverhead which reduce overall system capacity. Thus, the appropriatefrequency of the S1 periods will vary somewhat in accordance with theneeds of any particular system. In the embodiment shown, the S1 quietperiods are used to delimit the frames, although it should beappreciated that this is not a requirement. In general, the use of theS1 quiet periods will reduce the access time required to initiate acommunication. When appropriate, the use of DDPRs can further reduce theaccess time of the requesting remote unit.

As described above, initialization time intervals, S2, and retrainingtime intervals, S3, are not as numerous as the S1 quiet periods becauseinitialization and retraining usually do not demand as rapid a responseas a request for an immediate communications. In one embodiment, S2'sand S3's alternate every other super-frame 36. In yet anotherembodiment, S2's and S3's can be allocated dynamically by the centralunit 10 to adjust for changing circumstances. By way of example, more ofthe reserved time intervals 38 can be allocated as initialization timeintervals at times when remote units are more likely to be installed andrequire initialization, such as during the day. During the evening wheninstallations are less likely, more of the reserved intervals 38 can beallocated as retraining time intervals.

Referring next to FIG. 8, a transmitter 405 suitable for transmittingdigital signals in either polled or fast access transmission mode inaccordance with the present invention will be described. Transmitter 405is typically, but not necessarily, implemented as part of a remote modemfor facilitating transmission to the central unit. As seen therein,there is shown a lead 407 which carries a digital data from a digitaldata source. The digital data may be, by way of example, data from aremote unit. The digital data is received by a forward error corrector410, which encodes the signal with forward error correction in order tohelp insure that minor transmission errors will not ruin the quality ofthe received signal. It should be pointed out that the use of forwarderror corrector 410 is optional and may be omitted in some systems.Whether without or with optional error correction, the signal is thenpassed to a Serial to Parallel converter 411 which in turn feeds aencoder 412 which divides the signal into blocks of data that are to betransmitted on a multiplicity of sub-carriers. In the embodimentdescribed above, a separation of 4.3 kHz would be appropriate. If polledtransmission mode is in effect, encoder 412 may represent, for example,a QAM encoder. In one embodiment, a 16-point constellation QAM encoderis considered suitable. If transmission is via fast access transmissionmode, encoder 412 may represent, for example, a four-point constellationDifferential Quadrature Phase Shift Keying (DQPSK) encoder. By way ofexample, a suitable DQPSK encoder is described in J. Bingham's textentitled "Theory and Practice of Modem Design" published by J. Wiley &Sons (1988), which is incorporated herein by reference. In the describedexample, the control signal to effect switching between the polledtransmission and fast access transmission modes is also inputted to theencoder, although it should be appreciated that it could alternativelybe added at other locations as well.

From the encoder 412, the signal is passed to an inverse fast Fouriertransform (IFFT) encoder 414 which converts frequency domain signals tothe time domain. A suitable IFFT encoder is described in J. Bingham'sarticle entitled: "Multicarrier Modulation: An Idea Whose Time HasCome," IEEE Communication Magazine, May 1990, which is incorporatedherein by reference for all purposes. From the IFFT encoder, the signalis passed to a parallel to serial converter 416 to convert the parallelsignals back into the serial format to be outputted on lead 418.Parallel to serial converter 416 further includes a prefix inserterwhich adds a cyclic prefix to the signal prior to the parallel to serialconversion. By way of example, in a signal having 512 samples, a 40sample cyclic prefix has been found to work well.

Referring next to FIG. 9, a receiver 530 suitable for receiving digitalsignals transmitted by the transmitter 405 in either polled or fastaccess transmission mode in accordance with the present invention willbe described. Receiver 530 may be implemented, for example, within thecentral modem. As seen therein, the receiver 530 receives a digitalsignal on lead 539. The signal is first passed to a serial to parallelconverter 541 to convert the received digital signal to a plurality ofparallel signals. If cyclic prefix is added to the signal in thetransmitter, serial to parallel converter 541 preferably includes acyclic prefix stripper to remove any cyclic prefix added earlier. Theparallel signals are then passed to a fast Fourier (FFT) decoder 543which complements the IFFT encoder 414. Thus, the FFT decoder 543converts the time domain signal back into the frequency domain. Asuitable FFT decoder is also described in article cited above.

From the FFT decoder 543, the signal is passed to a multiplicity ofdecoders 545 which decode the signals in each of the sub-channels.Decoder 545 complements the encoders 412. If the polled transmissionmode is in effect, decoder 545 may represent, for example, a QAMdecoder. If transmission is via the fast access transmission mode,decoder 545 may represent, for example, a Differential Quadrature PhaseShift Keying (DQPSK) decoder. From the decoder 545, the signals arepassed to a parallel to serial converter 547. In one embodiment, thesignal from parallel to serial converter 547 is further passed on to anoptional forward error correction (FEC) decoder 548, which removes theforward error correction information if it was added at the tranmitterend.

While the present invention is mainly concerned with the manipulation ofupstream communications from the remote units to the central unit 10, norestrictions are placed upon the type of downstream communicationsapplicable to such a system. The downstream channel can utilize discretemulti-tone modulation similar to the modulation used for upstreamcommunication, or it may utilize other suitable techniques, such as,vesigial sideband (VSB). Also, the downstream channel can be furthercomprised of dedicated overhead channels for transmitting the relevantformatting signals, such as, but not limited to: S1, S2 and S3 flags,synchronization signals, and information about the allocation of thesub-channels 23. As appreciated by those skilled in the art, numerousother methods of transmission schemes can be applied to the downstreamchannel in relation to the present invention.

Although only a few embodiments of the present invention have beendescribed in detail, it should be understood that the present inventionmay be embodied in many other specific forms without departing from thespirit or scope of the invention. In view of the foregoing, it should beapparent that the present examples are to be considered as illustrativeand not restrictive, and the invention is not to be limited to thedetails given herein, but may be modified within the scope of theappended claims.

I claim:
 1. In a bi-directional data transmission system thatfacilitates communications between a plurality of remote units and acentral unit using a symbol-based discrete multitone transmission schemethat has a multiplicity of discrete sub-channels provided forfacilitating upstream communications between the plurality of remoteunits and the central unit, a method of informing the central unit ofthe transmission requirements of a remote unit, the method comprisingthe steps of:transmitting, using a fast access transmission mode, acommunication access request from a selected first remote unit to thecentral unit, the communication access request comprising a uniqueremote unit identifier identifying the selected first remote unit andbeing transmitted from the selected first remote unit on at least oneunused sub-channel using a modulation scheme that does not requireequalization to decode at the central unit; and allocating at least onesub-channel to the selected first remote unit in response to thecommunication access request for facilitating upstream communicationsbetween the selected first remote unit and the central unit.
 2. Themethod of claim 1 where said communication access request furthercomprises a data transmission request signal.
 3. The method of claim 2wherein the data transmission request is a two bit signal.
 4. The methodof claim 2 wherein the data transmission request is a defined datapacket request signal, and wherein the central unit allocates sufficientsub-channels to the selected first remote unit such that the selectedfirst remote unit can transmit a data packet in conformance with storeddefined data packet transmission requirements associated with theselected first remote unit, the stored defined data packet transmissionrequirements being known to the central unit prior to the receipt of thedefined data packet request signal.
 5. The method of claim 2 wherein thecentral unit allocates sufficient sub-channels to the selected firstremote unit until the selected first remote unit indicates that itdesires a change.
 6. The method of claim 2 wherein the modulation schemeis Differential Quadrature Phase Shift Keying.
 7. The method of claim 2wherein the unique remote unit identifier does not include groupidentifier information.
 8. The method of claim 1 wherein the selectedfirst remote unit monitors communications prior to transmitting thecommunication access request.
 9. The method of claim 1 furthercomprising the step of inhibiting remote units from transmitting usingthe fast access transmission mode responsive to a command from thecentral unit.
 10. The method of claim 9 wherein the command from thecentral unit is generated when usage on the system exceeds a predefinedthreshold.
 11. The method of claim 10 further comprising the step ofenabling remote units to transmit using a polled transmission moderesponsive to the command from the central unit.
 12. The method of claim1 further comprising the step of broadcasting a collision detectedmessage from the central unit to remote units when a garbledtransmission is received by the central unit on any unused sub-channel.13. The method of claim 1 further comprising the step of retransmitting,using the fast access transmission mode, the communication accessrequest from the selected first remote unit to the central unit if nosub-channel is allocated to the selected first remote unit at theexpiration of a predefined time period.